Position racks in the top and bottom thirds of the oven and heat the oven to 450°F. Butter a 9x13-inch (or similar) baking dish.

In a large bowl, toss the white mushrooms with 2 Tbs. of the oil, 1/2 tsp. salt, and 1/4 tsp. pepper. Spread the white mushrooms evenly on a large rimmed baking sheet. Toss the “wild” mushrooms with the remaining 2 Tbs. oil, 1/2 tsp. salt, and 1/4 tsp. pepper in the bowl and spread on a second large rimmed baking sheet. Roast, stirring the mushrooms and rotating the baking sheets once, until golden-brown and any liquid has evaporated, about 25 minutes. Reduce the oven temperature to 350°F.

Meanwhile, melt 2 Tbs. of the butter and pour into a medium bowl. Add the breadcrumbs and fontina and toss well.

In a 12-inch skillet, melt the remaining 2 Tbs. butter over medium heat. Add the leeks and 1 tsp. salt and cook, stirring often with a wooden spatula, until golden, 7 to 10 minutes. Add the vermouth and boil, stirring and scraping up any browned bits, until reduced by half, 1 to 2 minutes. Stir in the heavy cream, sage sprigs, 1/2 tsp. salt, and 1/4 tsp. pepper and bring just to a boil over high heat. Lower the heat to medium low and simmer for 5 minutes to infuse. Remove the pan from the heat and discard the sage sprigs.

Add the mushrooms and chopped sage to the leek mixture and toss to coat. Transfer to the prepared dish. Top with the breadcrumb mixture and bake in the top third of the oven until the breadcrumbs are golden-brown, about 30 minutes.

user reviews

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by CHACK504, 11/29/2015This was a hit at Thanksgiving. It is labor intensive. A few comments: Salt and pepper aren't listed in the ingredients but thankfully that is a pantry staple. The instructions don't say what the temperature of the oven should be when you put the assembled gratin in to brown. 450 degrees, which was used to roast the mushrooms, seems too hot to me. Also, the "wild" mushrooms cooked faster than the white mushrooms. For special occasions, I'd make this again.

by mams1, 2/6/2015Absolutely Divine!!!! A little labor intensive, but so worth it.. Rave reviews from entire family. This is a keeper.

by elo, 12/24/2012Outstanding! Everyone's favorite dish at Thanksgiving. So good that we're bringing it back for Christmas. Used a cheddar/fontina blend, which tasted great. Other than that, followed the recipe as-is - excellent!

by goandgo, 11/28/2012Fabulous side dish for lamb chops! Time intensive, but worth it!